Imaging devices use a variety of marking materials to generate a physical image of an electronic image. The materials include, for example, aqueous ink, melted ink, and toner. The marking material may be ejected onto or developed on an image receiving member. For example, electronic image data may be used to operate a raster to generate a latent image on a photoreceptor belt and then the latent image is developed with toner material in a development station. With aqueous ink or melted ink, a printhead ejects the melted ink onto an image receiving member, also known as an image drum. The inkjets in the printhead are operated by a printhead controller to eject ink onto the image receiving member. The printhead controller generates firing signals with reference to electronic image data to operate the inkjets.
Once the marking material is deposited onto an image receiving member, the image may be transferred or transfixed to an image media. For example, a sheet or web of image media may be moved into a nip formed between the image receiving member and a transfix or fuser roller so the image can be transferred to the image media. The movement of the image media into the nip is synchronized with the movement of the image on the image receiving member so the image is appropriately aligned with and fits within the boundaries of the image media. The pressure within the nip helps transfix or fuse the marking material onto the image media.
The image receiving member is typically heated to improve compatibility of the image receiving member with the inks deposited on the member. The image receiving member may be, for example, an anodized and etched aluminum drum or a steel drum. Within the drum, a heater reflector may be mounted axially within the drum. One or more heaters are located along the heater reflector. The heater reflector remains stationary as the drum rotates. Thus, the heaters apply heat to the inside of the drum as the drum rotates past the heaters on the reflector. The reflector helps direct the heat towards the inside surface of the drum.
Differences in temperatures of the components interacting during a print cycle cause thermal gradients to appear across the outside surface of the image drum. For example, the controller in one printer operates the heaters to maintain the temperature of the outside surface in a range of about 55 degrees Celsius, plus or minus 5 degrees Celsius. The ink that is ejected onto the print drum has a temperature of approximately 110 to approximately 120 degrees Celsius. Thus, images having areas that are densely pixilated may impart a substantial amount of heat to a portion of the print drum when several copies of such images are printed. Additionally, the drum experiences convective heat losses from the exposed surface areas of the drum as the drum rapidly spins in the air about the drum. The recording media contacting the print drum causes further heat losses on the surface of the drum. For example, paper placed in a supply tray has a temperature roughly equal to the temperature of the ambient air. As the paper is retrieved from the supply tray, it moves along a path towards the transfer nip. Typically, this path includes a media pre-heater that raises the temperature of the media. These temperatures may be approximately 40 degrees Celsius. Thus, when the media enters the transfer nip, areas of the print drum having relatively few drops of ink on them are exposed to the cooler temperature of the media. Consequently, densely pixilated areas of the print drum are likely to increase in temperature, while more sparsely covered areas are likely to lose heat to the passing media. These differences in temperatures result in thermal gradients across the print drum.
Efforts have been made to control the thermal gradients across a print drum for the purpose of maintaining the surface temperature of the print drum within the operating range. Heater control alone is sometimes ineffective because the amount of ejected ink in some images may raise the surface temperature of the print drum above the operating range even when the heater in that portion of the drum is off. In some print drums, a fan has been added at one end of a print drum to provide cooling. The print drum is open at each flat end of the drum. To best provide cooling, the fan is located outside the print drum and is oriented to blow air from the end of the drum at which the fan is located to the other end of the drum where it is exhausted. The fan is electrically coupled to the controller so the controller activates the fan in response to one of the temperature sensors detecting a temperature exceeding the operating range of the print drum. The air flow from the fan eventually cools the overheated portion of the print drum and the controller deactivates the fan.
While the fan system described above works for maintaining the temperature of the drum within an operating range, the system possesses some inefficiencies. Specifically, inefficiency arises when the surface portion of the print drum at which the air flow is exhausted has a higher temperature than the surface area near the end at which the fan is mounted. In response to the higher temperature detection, the controller activates the fan. As the cooler air enters the drum, it absorbs heat from the area near the fan that is within operating range. This cooling may result in the controller turning on the heater for that region to keep that area from falling below the operating range. Even though the air flow is heated by the region near the fan and/or the heater in that area, it is still able to cool eventually the overheated area near the drum end from which the air flow is exhausted. Nevertheless, the energy spent warming the region near the fan and the additional time required to cool the overheated area with the warmed air flow from the fan adds to the operating cost of the printer.
The above system is generally limited to image drums made of anodized aluminum because of its high thermal diffusivity. Materials with lower thermal diffusivity, such as steel, cannot be used efficiently as the temperature may increase beyond the operating temperature when ink is concentrated at a certain location on the drum. Therefore, more efficient cooling of the print drum is desired.